Recently, automatic guidance systems for agricultural and construction vehicles had been devised to steer these vehicles through agricultural fields or work sites along predetermined paths. These guidance systems determine the path and position of the vehicle by combining several different sensor signals and using the signals to determine the position of the vehicle in the field and its distance from the desired track it should be traveling. Clearly, if the vehicle is to follow a predetermined path through the field it must know its current position.
Unfortunately, there is no one universal sensor that is capable of indicating vehicle position with enough accuracy to permit the vehicle to be guided by that signal alone. Consequently, agricultural and construction vehicles using automatic guidance systems combine several different sensor signals which they weigh and balance in a variety of ways.
Each sensor has particular characteristics that make it good to use in certain situations, but bad to use in others. Some sensors are quite good at providing absolute position. Others are quite good at providing relative position. For example, GPS sensors are quite good at providing an absolute position that is accurate to within 20-100 feet of the true position. Unfortunately, GPS sensors have a certain amount of random position “noise” that prevents them from being more accurate than this. To determine position any more accurately than this requires additional sensors.
An alternative and refined GPS system often called “differential GPS” or “DGPS”) provides a more accurate measurement of position. It, too, suffers from the same random noise problems that standard GPS systems do. For purposes of this document, the term “GPS” shall refer to both GPS and DGPS systems.
GPS sensors can only provide data indicative of a vehicle's orientation when the vehicle is moving. Once the vehicle is stopped, the GPS sensor has no way of determining what direction the vehicle is pointed. Additional sensor input is required to make this determination.
One such additional sensor that can be combined with the GPS signal is a yaw rate sensor. A “yaw rate” is the rate at which a vehicle turns to the left or the right. By monitoring a yaw rate sensor, a controller can determine the direction the vehicle is pointed, even when it is standing still. Unfortunately, yaw rate sensors tend to drift over time. In other words, even when standing still the sensors indicate some slow turning to the left or right.
Another sensor that is useful in navigating vehicles through fields and can supplement the GPS sensor signal is the electronic compass. These compasses are sensitive to changes in the Earth's magnetic field. Thus, when one turns an electronic compass, it senses the changing magnitude of the magnetic field and responsively determines the direction the compass is pointed. These compasses are much more accurate over time, as compared to yaw rate sensors. They lack the precision of yaw rate sensors, however, and therefore do not provide an accurate measurement of vehicle heading.
Another sensor that is helpful in determining the position of the vehicle and can supplement the GPS sensor signal is a vehicle speed sensor. These sensors indicate the speed at which the vehicle is traveling over the ground. These sensors are highly accurate at measuring distance, but (depending upon their design) provide no indication whatsoever of the direction of travel. Inexpensive speed sensors merely provide a numeric value that indicates the rotational speed of a wheel or shaft on the tractor, and not its direction. In other words, the sensor provides a numeric value that tells the controller how fast the vehicle is moving, but does not tell the controller the direction of movement. The vehicle might be moving forward at the speed indicated by the speed sensor, or it might be moving in reverse.
The advantage of the vehicle speed sensor is its accuracy over short distances. Over distances of 100 or 200 feet, a vehicle speed sensor can provide an extremely accurate measure of distance traveled. Over long distances of a mile or more, a vehicle speed sensor cannot provide an accurate measure of distance traveled. However, a GPS provides reasonably good accuracy at larger distances, so a combination of these two sensors is particularly suitable to determining vehicle position.
A navigation system using a GPS and a vehicle speed sensor also needs to know the direction of travel in order to make sense of the vehicle speed sensor signal. One of the most common direction “sensors” is the FNR lever found in the operator's compartment of many agricultural and construction vehicles. “FNR” stands for “forward-neutral-reverse”.
The FNR lever enables the operator to shift from a forward direction of travel to a reverse direction of travel by moving the FNR lever from its F (forward) position to its R (reverse) position. The FNR lever permits the operator to shift from reverse direction of travel to a forward direction of travel by moving the FNR lever from its R position to its F position. The operator can stop the vehicle entirely by moving the FNR lever to its N or “neutral” position. Of course, the FNR lever alone does not move or stop the vehicle. The FNR lever functions as an input device to a transmission controller that engages and disengages the vehicle's transmission in response.
By combining these two signals—the speed signal indicating the absolute speed of the vehicle and the direction of vehicle travel indicated by the FNR lever—and automatic guidance system (in combination with the other sensor signals) can determine accurately the vehicle's position.
While the FNR lever is a good estimation of the direction of travel of the vehicle, it does not always immediately reflect the direction in which the vehicle is traveling. For example, many construction and agricultural vehicles permit the operator to shift the FNR lever from forward to reverse (and vice versa) while the vehicle is moving.
A work vehicle can be traveling forward at a top speed of 10 to 20 miles an hour, for example, and the operator can move the FNR lever to reverse. These vehicles have intelligent transmission controllers that are programmed to rapidly slow the vehicle down until it is stopped, then to start the vehicle moving in reverse, accelerating it until it reaches its predetermined reverse speed.
This process of automatic shifting from forward to reverse while moving forward by slowing the vehicle to a stop, then reversing direction of travel, then accelerating up to speed in reverse is called “shuttle shifting”. It is possible to do in a class of transmissions generally called “power shift” or “shuttle shift” transmissions. These vehicles have one or more clutches that can be engaged or disengaged by hydraulic or mechanical actuators under computer control. The computer—typically called a transmission controller—is coupled to the speed sensor and to each of the clutches to selectively engage and disengage them as the controller deems appropriate in accordance with its internal programming.
To perform a typical shuttle shift, a transmission controller is typically programmed to immediately disengage the transmission from its forward gear ratio. The transmission controller does this by automatically disengaging the clutches necessary to drive the vehicle in a forward direction (these clutches are known as the “forward clutches”). The transmission controller is programmed to then slow the vehicle down by automatically engaging the clutches necessary to drive the vehicle in a reverse direction (these clutches are known as the “reverse clutches”). The transmission controller does not fully engaged the reverse clutches, however. Instead, it feathers the reverse clutches, partially engaging them, and permitting them to dissipate the vehicle's energy and to slow the vehicle to a stop. Once the vehicle is stopped—or close to stopped—the transmission controller fully engages the reverse clutches and the work vehicle accelerates in reverse.
This period of shuttle shifting can be significant. For a work vehicle going 18 or 20 miles an hour with a heavy load, it can take four seconds from the time the operator moves the FNR lever from the F position to the R position before the vehicle slows to a complete stop. During this four second period until the work vehicle is stopped, it might travel as far as 200 feet. During this four second period, the FNR lever is essentially providing a false direction of travel to the automatic guidance system, possibly leading the guidance system to steer the vehicle in a wrong direction. To understand how this can occur, it is helpful to understand how navigation controllers that implement the automatic guidance system function.
A typical automatic guidance system is embodied as a program executed by digital computer-based navigation controller. The navigation controller is connected to the various sensors, GPS, yaw rate sensor, speed sensor, electronic compass, FNR lever and whatever other devices or sensors provide it with a signal indicative of position, orientation, and movement. The navigation controller combines the sensor signals by a variety of sensor fusion techniques and determines the vehicle's corresponding position, speed, and direction of travel. Having determined the vehicle's position, the navigation controller then determines how to steer the vehicle onto a desired path of travel.
For an agricultural tractor, for example, the desired path may be along a crop line in a field. For a road grader, the path may follow a contour line of a hill. Whatever the path, the function of the guidance system is to determine the appropriate path, determine the position of the vehicle, calculate the appropriate steering action, and drive the steering actuators to turn the vehicle back onto the desired path.
A navigation controller typically calculates a new control action every 10 ms, or about 100 times a second. What this means is that 100 times a second a typical navigation controller will read its sensors, combine them according to its own internal programming, and determine the appropriate amount to steer the wheels. If the navigation controller makes an error in its calculations, the work vehicle can turn unexpectedly and leave the desired path of travel. The distance from the desired path of travel or “track” is called the “cross-track error”. Cross-track error is a fundamental measure of the performance of a n automatic guidance system. Anything that tends to let the vehicle turn away from the track and increase the cross-track error is undesirable. Anything that tends to reduce the cross-track error is beneficial.
False readings from the FNR lever are one source of cross-track error in many automatic guidance systems. Using the shuttle shifting example above, assume an operator, traveling at 18 miles an hour in a forward direction, shifts the FNR lever from forward to reverse.
As explained above, the vehicle does not immediately go into reverse. Instead, it continues forward for perhaps 200 feet (four seconds) slowing the vehicle to a stop before it starts to travel in reverse. During this four second interval, therefore, the navigation controller has received 400 (100 per second times four seconds) false readings of the direction of travel. The FNR lever “told” the navigation controller that the vehicle was traveling in reverse, yet the vehicle continued traveling forward a substantial distance (and time) before truly changing its direction.
Whenever this happens, the navigation controller is unable to process the direction signal given by the FNR lever and properly combine it with the GPS signal. This is understandable, since the successive GPS readings would indicate the vehicle is continuing to travel forward, while the successive FNR lever readings indicate that the vehicle is traveling backwards in the exact opposite direction. Rather than benefiting from the FNR direction signal to more precisely estimate the vehicle's position, speed, orientation, and direction of travel, the navigation controller is led astray, calculating a mistaken and erroneous control signal and possibly driving the wheels to a wrong position, steering the vehicle away from the desired path of travel.
What is needed, therefore, is a work vehicle with an improved system for guidance. What is also needed is an improved method for determining vehicle travel direction for an automatic guidance system that more accurately indicates the direction of travel of the vehicle than an FNR lever. What is also needed is a control system that better determines the vehicle's direction of movement. It is an object of this invention to provide such a work vehicle, method and control system.
These and other aspects of the invention will become apparent upon review of the detailed description, the figures, and the claims provided below.